Scanning radiographic equipment differs from conventional radiography in that it employs a narrowly collimated beam of radiation, typically x-rays formed into a fan or pencil beam, rather than a broad area cone beam. The small beam size used in scanning radiographic equipment allows replacement of an image forming sheet of radiographic film, used with conventional radiographic equipment, with a small area electronic detector element or array of such elements.
The detector elements receiving the transmitted radiation produce electrical signals which may be converted to digital values by an analog to digital converter for the later development of an image or for other processing by computer equipment. The ability to quantify the measurement of the transmitted radiation, implicit in the digitization by the analog to digital converter, allows not only the formation of a radiographic "attenuation" image but also the mathematical analysis of the composition of the attenuating material by dual energy techniques. See generally, "Generalized Image Combinations in Dual KVP Digital Radiography", by Lehmann et al Med Phys 8(5) Sep./Oct. 1981.
Such dual energy techniques quantitatively compare the attenuation of radiation at two energies to distinguish, for example, between bone and soft tissue. Dual energy techniques allow the measure of bone mass, such measurement being important in the treatment of osteoporosis and other bone diseases.
The limited area of the beam of radiation used in scanning radiographic systems requires that the beam be moved over an area, if a conventional image is to be formed. Typically, the pencil or fan beam will be scanned in a raster pattern over the area to be measured, each line of the scan separated by the width of the pencil or fan beam, with the directions of scanning being generally perpendicular to the direction of the radiation.
Images formed by a scanning radiographic system are potentially more accurate than those produced by a typical broad beam radiograph system. This accuracy arises from the limited divergence of the rays of the pencil or fan beam from the principal axis of the radiation, as compared to a broad area cone beam. This narrow collimation of the pencil or fan beam reduces "parallax" in the projected image, potentially providing extremely accurate morphological measurements of certain structures such as the vertebrae in the spine. Such morphological measurements are used to evaluate various dimensions of a vertebra to detect crushing or other deformation that are one element of certain bone diseases such as osteoporosis. See e.g. Minne et al., "A Newly Developed Spine Deformity Index (SDI) to Quantitate Vertebral Crush Factors in Patients with Osteoporosis," Bone and Mineral, 3:335-349 (1988); J. C. Gallagher et al, "Vertebral Morphometry: Normative Data," Bone and Mineral, 4:189-196 (1988); Hedlund et al, "Vertebral Morphometry in Diagnosis of Spinal Fractures," Bone and Mineral, 5:59-67 (1988); and Hedlund et al, " Change in Vertebral Shape in Spinal Osteoporosis," Calcified Tissue International, 44:168-172 (1989). Automatic techniques for morphological measurements of bone are described in U.S. patent application Ser. No. 07/944,626 filed Sep. 14, 1992 and entitled: "Method for Analyzing Vertebral Morphology Using Digital Radiography" assigned to the same assignee as the present application and hereby incorporated by reference
In order to make accurate morphological measurements and to provide clinically valuable dual energy measurements of a variety of body structures, the radiation source and detector should be easily positioned at different angles about the patient. Further, at each such angle, the radiation source and detector must have the necessary clearance from the patient to perform the required scanning. Because the scanning can take some length of time, the patient must be fully supported, typically in a supine position, and yet ideally the patient support must be constructed so as to not unduly interfere with the attenuation measurements made during the scan. Finally, the positioning of the patient with respect to the radiation source and detector should be such as to maximize the quality of the morphological data obtained.